1. Field of the Invention
The present invention relates to an image reading apparatus, multifunction printer apparatus, and image processing method. Particularly, the present invention relates to an image reading apparatus, multifunction printer apparatus, and image processing method which correct density or brightness represented by image data obtained by optically reading an image original.
2. Description of the Related Art
A color scanner is known as an image reading apparatus which reads an image by switching light of different light emission wavelengths. Such a color scanner has a linear light source and an image sensor provided on a carriage movable in a predetermined direction. The light source includes LEDs capable of irradiating light emission wavelengths corresponding to three primary colors of light, that is, red (R), green (G), and blue (B). The carriage is moved in a direction (sub-scanning direction) perpendicular to the elongated direction (main scanning direction) of the linear light source. The liner image sensor receives reflected light obtained by irradiating an image original with light and reads the image original. To read the image original, a scanning read method is employed.
In the scanning read method, an original is read by switching three LEDs serving as a light source while conveying a CIS (Contact Image Sensor) unit in the sub-scanning direction. More specifically, the R component data of a color image is obtained by lighting a red LED. Next, the G component data is obtained by lighting a green LED. Finally, the B component data is obtained by lighting a blue LED. Image data of one line is obtained in one red, green, and blue LED lighting cycle. Image data of one page of the image original is obtained by repeating the lighting cycle while conveying the CIS unit in the sub-scanning direction.
In scanning read in which the red, green, and blue LEDs are sequentially turned on, color misalignment occurs. As a method of reducing color misalignment, a method of performing reading by turning on two LED light sources between charge readout timings is known, as disclosed in Japanese Patent Laid-Open No. 2005-184390.
If bright LEDs are used to improve the signal-to-noise ratio, the cost increases. To solve this problem, a method of reading an image by simultaneously turning on two LED light sources is known, as disclosed in Japanese Patent Laid-Open No. 2006-197531.
Alternatively, an image forming apparatus described in Japanese Patent No. 3750429 is known, which performs a reading operation appropriate for an original by switching illumination light in accordance with the original type such as a negative original or positive original.
FIG. 12 is a timing chart showing a primary color reading method of reading an image original by lighting only one color LED at a single timing.
As shown in FIG. 12, according to this method, the red (R), green (G), and blue (B) LEDs are sequentially turned on so that the respective color component data are output in synchronism with a pulse signal SH. When the red LED changes from ON to OFF, and the pulse signal SH is turned on, R component data is output. Similarly, when the green LED or blue LED changes from ON to OFF, and the pulse signal SH is turned on, G component data or B component data is output.
Let (R,G,B)=(255,255,255) be the brightness value of a read white original, and (R,G,B)=(0,255,255) be the brightness value of a read cyan original.
When an edge at which an original changes from white to cyan is read at the timing shown in FIG. 12, the output data of a line (a) is (R,G,B)=(255,255,255), and the output data of a line (b) is (R,G,B)=(0,255,255). In the line (a), at the light emission timing of the red LED, the original color is white. Hence, the brightness output value of the R component is 255. At the light emission timings of the green and blue LEDs, the original color is cyan. Hence, the brightness output value of the G component is 255. The brightness output value of the B component is also 255.
FIG. 13 is a timing chart showing a complementary color reading method of reading an image original by simultaneously turning on two color LEDs (simultaneously lighting two primary colors).
When an edge at which an original changes from white to cyan is read by the complementary color reading method at the timing shown in FIG. 12, the brightness output values are as follows. The output data of a line (c) is (RG,GB,BR)=(510,510,255), and the output data of a line (d) is (RG,GB,BR)=(255,510,255). The read data is converted into the brightness values of the R, G, and B color components by equation (1). In the line (c), (R,G,B)=(128,255,128). In the line (d), (R,G,B)=(0,255,255).
                              (                                                    R                                                                    G                                                                    B                                              )                =                              1            2                    ⁢                      (                                                                                -                    1                                                                    1                                                  1                                                                              1                                                                      -                    1                                                                    1                                                                              1                                                  1                                                                      -                    1                                                                        )                    ⁢                      (                                                            GB                                                                              BR                                                                              RG                                                      )                                              (        1        )            
Using the obtained values of the lines (a) to (d), a CTF (Contrast Transfer Function) is calculated by equation (2). In the primary color reading method, CTF=18%. In the complementary color reading method, CTF=7%.
                    CTF        =                                                            W                p                            -                              B                p                                                                    W                p                            +                              B                p                                              ·          100                                    (        2        )            
Note that, in equation (2), Wp is the maximum brightness, and Bp is the minimum brightness.
As understood from a comparison between the calculated CTFs, the CTF value obtained by the complementary color reading method is smaller than that obtained by the primary color reading method. That is, if the complimentary reading method is employed, an image having a blurred edge is read. Since the reading method in the sub-scanning direction is different between the primary color reading method and the complementary color reading method, the CTF value readily changes at an edge portion in the sub-scanning direction.
The same as in image original reading by the method of simultaneously lighting two colors also applies to a two-color LED time-divisional lighting method as shown in FIG. 14.
The color of the original medium itself (to be referred to as a “background color” hereinafter) is generally perceived as a single color. At close range, however, a fine change in the color is recognizable. This can be regarded as extremely small stains generated in the process of manufacturing original printing media themselves due to various factors such as a thickness variation or density unevenness of printing media, or clusters of non-uniformly distributed pulp. In other words, a printing medium cannot have a completely uniform background characteristic. In fact, minute granular non-uniform portions exist. This structure will be referred to as a “minute stain” in the following explanation.
The minute stains exist at random all over a printing medium. When reading an original printed on such a printing medium, the background color is differently read between the primary color reading method (to be referred to as “primary color reading” hereinafter) described with reference to FIG. 12 and the complementary color reading method (to be referred to as “complementary color reading” hereinafter) described with reference to FIG. 13.
This phenomenon will be described below in more detail.
The background color should have the same signal value independently of the original reading method such as primary color reading or complementary color reading.
Primary color reading will be described first. In this case, each LED emits light during only ⅓ the unit time per pixel to be read. Data of one pixel is read only when all LEDs have emitted light.
This will be described with reference to the accompanying drawings.
FIG. 15 is a schematic view showing reading of an arbitrary pixel.
In a U region shown in FIG. 15, the red LED emits light to irradiate an original so that a first signal value R is obtained. In an M region, the green LED emits light to irradiate the original so that a second signal value G is obtained. In a B region, finally, the blue LED emits light to irradiate the original so that a third signal value B is obtained. As a result, the pixel of interest shown in FIG. 15 is represented by three, first to third signal values R. G, and B.
Assume that a “minute stain” exists in the U region in FIG. 15, as shown in FIG. 16. The stain in the U region is read by the red LED. In primary color reading, the signal value of the read U region represents the R component data of all the U, M, and B regions. In other words, the influence of the “minute stain” in the U region appears in only the R component data.
Hence, letting R′ be the signal value by the red LED containing the influence of the “minute stain”, and δr be the signal difference generated by the influence of the “minute stain”, δr=R−R′. Similarly, when a “minute stain” is read in the M or B region, a difference δg or δb is generated for the second or third signal value. The differences δr, δg, and δb will be collectively described as δ hereinafter. Even when a background color is generally recognized as almost the same color, a variation of about δ is generated in each pixel in accordance with minute stains on the original, as is apparent from the histograms of the first to third signal values.
In complementary color reading, each LED emits light during ⅔ the reading time per pixel. That is, two LEDs read the U region in FIG. 15. A case in which a “minute stain” exists in the U region, as shown in FIG. 16, will be examined as in the case of primary color reading.
In this case, both the red LED and the green LED suffer the influence of the “minute stain”. A signal value obtained by complementary color reading has a brightness range twice that of a signal value obtained by primary color reading.
To equalize the brightness ranges, the signal values obtained by the red and green LEDs must be halved. As a result, the signal difference δ generated by the influence of the “minute stain” in the read signal value appears as δ/2 in both the red and green LEDs. Similarly, when a “minute stain” is read in the M or B region, the signal difference δ/2 is generated for each of the combination of the green and blue LEDs and the combination of the blue and red LEDs.
Even when a background color is generally recognized as almost the same color, a variation of about δ/2 is generated in each pixel in accordance with minute stains on the original, as is apparent from the histograms of the first to third signal values.
As is apparent from the above examinations, in primary color reading and complementary color reading, a signal value obtained by reading a background color varies by δ−δ/2=δ/2.
This is shown in FIG. 17.
FIG. 17 shows the histograms of brightness values obtained by reading the background color of a printing medium by primary color reading and complementary color reading. Referring to FIG. 17, the abscissa represents the signal value near the background color, and the ordinate represents the appearance frequency. The solid line indicates a variation in the signal value near the background color in primary color reading. The broken line indicates a variation in the signal value near the background color in complementary color reading.
To completely remove the background color of an obtained image, the tail portions of the histograms in FIG. 17 must be removed. If the tail portions remain, discomfort isolated points are generated in the background of the read image.
If primary color reading or complementary color reading is available as the under color removal means, one of these methods cannot perform appropriately under color removal. Additionally, for the same under color removal amount, one of the methods cannot perform appropriate under color removal.